The Focal Properties and Spherical Aberration Constants of Aperture Electron Lenses

The focal properties and spherical aberration constants of a number of aperture electron lenses of varying diaphragm thickness and varying strength were determined numerically using potential distributions obtained ( a ) from a rigorous solution due to Ollendorff, and ( b ) on a resistor network analogue The focal lengths of `weak' aperture lenses of finite diaphragm thickness agree closely with those given by Davisson and Calbick's formula for the `infinitely' thin aperture lens; `strong' aperture lenses of all thicknesses show systematic deviations from this formula The primary spherical aberration constants of diverging aperture lenses are small, and of such sign that they cannot be used for the correction of the spherical aberration of converging electron lenses.